Traction control systems help to prevent or limit a vehicle's wheels from slipping during acceleration on different surfaces. Traction of a vehicle is established as its wheels contact a surface so that when the wheels are rotated, usually by a driving force, the vehicle will be moved along the surface in a desired direction. The combination of the coefficient of friction and the force exerted by a wheel against the surface produces traction. When the coefficient of friction of the surface is less than the force exerted, the wheel will slip during acceleration of the vehicle, adversely affecting acceleration performance and driving stability. Slippage can occur as a result of excessive accelerative forces applied to vehicle wheels or inadequate wheel to surface friction that can be present with wet or icy conditions. Once the condition is recognized, a vehicle driver, particularly in an automobile or like vehicle, may try to control slippage by reducing engine power or by applying the brakes, both of which can reduce the speed at which a drive wheel is rotating. The driver may not be aware that slippage is occurring, however, and may not be able to take corrective action as quickly as required.
Aircraft are required to travel along the ground between landing and takeoff, and traction control during taxi can be a challenge, particularly under adverse weather or runway conditions. Aircraft ground travel is presently conducted by using thrust from the aircraft's main engines and/or tow vehicles to move aircraft between runways and gates after landing and prior to takeoff. Apart from the application of the aircraft's brakes, effective and reliable traction control has not heretofore been possible during this type of aircraft ground movement. Moving an aircraft on the ground without the use of a tug or tow vehicle or the aircraft's engines has been proposed. For example, U.S. Pat. No. 7,891,609 to Cox et al, owned in common with the present application, describes moving an aircraft along taxiways using at least one self propelled undercarriage wheel. McCoskey et al describes a powered nose aircraft wheel system useful in a method of taxiing an aircraft that can minimize the assistance needed from tugs and the aircraft engines in U.S. Pat. No. 7,445,178. Neither the need for traction control nor a system that reliably controls traction in an aircraft during ground travel is suggested in either of these patents.
Various traction control systems have been proposed for automobiles and like vehicles to automatically adjust traction between the vehicle's drive wheels and the road or ground surface to minimize acceleration slip. These include, for example, systems that control traction using braking force adjustment, engine throttle control, and engine fuel supply control. Other traction control systems for automotive use have also been proposed. In U.S. Pat. No. 6,002,979 to Ishizu (Nissan), for example, an automobile traction control system in combination with a fuel supply system that adjusts driving torque delivered to each drive wheel by adjusting engine power is described. This system monitors slipping of a drive wheel with respect to a target drive wheel speed and includes engine control means that cooperates with a fuel supply system to decrease engine power by decreasing fuel supplied in response to a detected slipping condition. This system is sensitive to preventing engine stall when the speed of the drive wheel is reduced to a target drive wheel speed. A plurality of sensors is employed to assist with the electronic control of the Ishizu traction control system.
The traction control device disclosed in U.S. Pat. No. 6,007,454 by Takahira et al (Toyota) automatically detects slipping conditions of each of the pairs of wheels in a four wheel drive automobile by comparing the mean rotational speed of the front or rear drive wheels to a threshold value. A brake system is electronically controlled to apply brakes to at least one of the pairs of front or rear wheels, thereby executing traction control according to a selected gear transmission ratio. When optional vehicle speed sensors are included in this system and wheel rotation speeds are compared to vehicle speed, the automobile's engine can be controlled to decrease rotational power output. Neither of the systems described in the aforementioned patents is disclosed to have any utility beyond automotive applications or under conditions likely to be encountered by taxiing aircraft, however.
Traction control systems useful in hybrid and electric vehicles, primarily automobiles, have also been proposed. U.S. Pat. No. 5,450,324 to Cikanek (Ford), for example, discloses a combined traction control and antiskid braking system operatively connected to an electric traction motor and a hydraulic braking system. Present vehicle parameters are monitored by sensors, and a processor responsive to the sensors calculates vehicle parameters not directly measurable to determine whether the vehicle state requires traction control or braking control. A control strategy based on the determined vehicle state is used by the processor to provide commands to a motor controller to control operation of the electric traction motor by reducing motor torque if traction control is appropriate or, alternatively, to a brake controller if hydraulic or regenerative antiskid braking control is needed. The main focus of the traction control and braking system disclosed by Cikanek is to maximize regenerated kinetic energy during braking and minimize kinetic energy loss due to wheel slip, primarily to overcome battery energy storage limitations. The application of this system to hybrid and electric vehicles or conditions beyond those described is not suggested.
The traction control system described in U.S. Pat. No. 6,577,944 by Davis uses existing engine speed sensors to determine the occurrence and degree of wheel slippage by comparing whether two successive engine speed readings exceed a selected threshold and generates an automatic proportional corrective action from the vehicle's engine, braking system, or both. This system is designed primarily for automotive internal combustion engines and/or electric motors. Nonautomotive uses in which a drive unit applies torque to a rotating component that must overcome resistance, such as in a turbine rotated by an electric drive motor and in a power boat with an internal combustion engine-driven propeller or screw, are also contemplated. As with the systems discussed above, the utility of this traction control system is limited primarily to automotive applications, and the control of traction in aircraft ground travel is not even remotely suggested.
A need exists for a traction control system designed to enable aircraft equipped with ground movement drive systems to be driven effectively and reliably during autonomous ground travel under a variety of weather and ground surface conditions without reliance on the aircraft's brakes or limitation by ground surface condition. The prior art fails to provide such a system.